The invention relates to an axial fan, whose fan blades are fixed to the rotor of an electrical driving motor and are mounted coaxially within a pipe section and extend over at least half the axial length of the said pipe section, whereby the flow channel formed by the pipe section is constricted in the flow direction from a wide inlet on the suction side to a necked-down portion and from the latter is widened to a wide outlet on the pressure side.
Only a limited space is available for axial fans of this type and it is therefore a problem of the invention to obtain within the space a maximum delivery efficiency in practical operation, accompanied by a minimum noise development.
According to the invention, this problem is solved through the necked-down portion closely embracing the periphery of the blades and extending only over a small fraction of the axial length of the blades on the suction side and through the shaft of a rotor being mounted with axial clearance in pivot bearings and being supported on the suction side is supported on a punctiform bearing centrally arranged in a flanged wheel and through the flanged wheel being fixed with spokes to the suction side edge of the pipe section, whereby the said spokes span from the annular axial surface of projection of the blades hollowed out by the flanged wheel to the suction side edges of the blade and namely with such an orientation of the inclined position that the axial projection of the intersection point of the spokes with the edges are displaced outwardly during operation.
As a result of the invention, it is possible to obtain a relatively large pressure side extension of the flow channel in the limited space available and in this area it is also unnecessary to narrow down the flow channel through the elements necessary for the mounting support. This is achieved by the suction side arrangement of the axially short necked-down portion and the suction side arrangement of the flanged wheel and spokes necessary for the mounting. With the object of obtaining a high delivery efficiency, it is advantageous to make the blades sufficiently long in the axial direction that the whole space available in the flow channel within the pipe section is used to the maximum. If this is done, the rear edges of the blades slide past the spokes in close proximity thereto. The resulting noise can be avoided by the special inclined orientation of the spokes.
Thus, an axially wide extension on the pressure side (outlet), almost up to the safety clearance at the outlet plane E.sub.A, which is made fully utilizable by the non-circular cross-sectional extension described hereinafter.
The suction side supported punctiform bearing of the rotor shaft provided in this connection has proved very unprone to wear in practical operation, because the reactive forces acting on the rotor shaft are directed towards the suction side and can be supported in such a punctiform bearing with extremely little friction. This bearing is particularly advantageous in cases where the fan is positioned to blow vertically upwards, thus, not only the reactive forces but also the weight of the rotor must be supported in the direction of the suction side. This leads not only to a relatively long service life but also a relatively short start-up time.
According to a further development of the invention, the section side constriction of the flow channel takes place with a continuously decreasing pitch in the flow direction and the pressure side extension is in the form of a diffuser. The outlet of the flow channel is bounded by a polygonal contour in whose corner areas the outlet-side end portion of the flow channel projects beyond the circular periphery circumscribed by the polygonal contour. As a result of this further development, the flow channel cross-section is further improved within the existing volume relationships and in support of the pressure side extension it is also possible to utilize the corner areas and the outlet side end portion of the flow channel obtains a non-circular cross-section, which counteracts an efficiency-reducting twisting of the blown out flow. The mass production of such pipe sections is facilitated if the outlet cross-section of the flow channel in the corner areas is limited by segments of a surface of revolution coaxial to the rotor axis. The mould faces corresponding to these segments can then be shaped on the lathe.
These non-circular cross-sectional extensions of the diffuser channel into the corners of the casing appropriately first pass to the final portion (e.g. the last quarter) of the axial casing, considered in the flow direction.
The degree of development of this corner extension also determines the amount of the above mentioned so-called torque recovery (or reduction of said twisting) obtained therewith and improves the fan characteristics.
In the said construction, on the discharge side in each centre of the contour sides, the diffuser is sectionally undisturbed until in the outlet plane, from where there is a peripherally increasing widening (preferably in a monotonic manner) up to the two neighbouring corner areas.
According to a further development of the invention, the flow channel inlet is bounded by a polygonal contour, in whose corner areas the inlet end portion of the flow channel projects beyond the circular periphery circumscribed by the polygonal contour.
As a result of this further development, the inlet is further widened by a non-circular cross-section on the suction side and the flow into the fan is improved.
The polygonal contour on the suction or pressure sides in many cases is due to a built-in flange which there seals the pipe section in the peripheral direction and whose outermost corners project over the flow channel and can have fixing elements e.g. holes.
It is recommended to arrange each spoke opposite an associated corner area and to fix the same to the peripheral end of this corner area located in the direction of rotation at the edge of the pipe section. As a result, the noise caused by the spokes is minimal.
The electric motor is preferably an external rotor-type motor, whose rotor surrounds the stator.
These and further objects, features and advantages of the present invention will become more obvious from the following description with reference to the accompanying drawings, which show for purposes of illustration only several embodiments of the present invention.